If Mal and Cobb grew old together in the deep dream before, why were they still young when they killed themselves on the rail track?

They were in fact old when they laid on the train track, but at the time when Cobb was describing the story to Ariadne, he was describing it without giving the detail that they were old. Thus what was shown was the illusion given by Cobb to Ariadne.

At the point Cobb tells Ariadne, he has forgotten that Mal and he had reached old age together. We see him remember this near the end, during the conversation with his projection of Mal in Level 4 (the "ruined city"). Her existence as a projection, and so as part of his own subconscious, is made clear when Cobb tells her she is his own imperfect creation. He then reminds her that his promise to her has been fulfilled: they did grow old together. As she is part of his subconscious, Cobb is basically reminding himself of this. Only now do we see them as an elderly couple.

They were in fact old when they laid on the train track, but at the time when Cobb was describing the story to Ariadne, he was describing it without giving the detail that they were old. Thus what was shown was the illusion given by Cobb to Ariadne.

They were in fact old when they laid on the train track, but at the time when Cobb was describing the story to Ariadne, he was describing it without giving the detail that they were old. Thus what was shown was the illusion given by Cobb to Ariadne.

At the point Cobb tells Ariadne, he has forgotten that Mal and he had reached old age together. We see him remember this near the end, during the conversation with his projection of Mal in Level 4 (the "ruined city"). Her existence as a projection, and so as part of his own subconscious, is made clear when Cobb tells her she is his own imperfect creation. He then reminds her that his promise to her has been fulfilled: they did grow old together. As she is part of his subconscious, Cobb is basically reminding himself of this. Only now do we see them as an elderly couple.

They were in fact old when they laid on the train track, but at the time when Cobb was describing the story to Ariadne, he was describing it without giving the detail that they were old. Thus what was shown was the illusion given by Cobb to Ariadne.

At the point Cobb tells Ariadne, he has forgotten that Mal and he had reached old age together. We see him remember this near the end, during the conversation with his projection of Mal in Level 4 (the "ruined city"). Her existence as a projection, and so as part of his own subconscious, is made clear when Cobb tells her she is his own imperfect creation. He then reminds her that his promise to her has been fulfilled: they did grow old together. As she is part of his subconscious, Cobb is basically reminding himself of this. Only now do we see them as an elderly couple.